1. Field of the Invention
This invention relates to the recycle of defective fuel pellets having a composition of UO.sub.2 alone or UO.sub.2 containing an oxide of plutonium, gadolinium or erbium into the manufacture of new fuel pellets. Particularly this invention relates to a method for comminuting defective fuel pellets through oxidation to fuel particles of U.sub.3 O.sub.8 alone or U.sub.3 O.sub.8 containing an oxide of plutonium, gadolinium or erbium, adding a sintering aid to the sintering powder which consists of said recycled fuel particles and fresh fuel powder of UO.sub.2 alone or UO.sub.2 in a mixture of PuO.sub.2, Gd.sub.2 O.sub.3 or Er.sub.2 O.sub.3, mixing the sintering powder uniformly, pressing the sintering powder into green pellets, and sintering green pellets in a reducing atmosphere to make new fuel pellets.
2. Description of Prior Art
The common method used to fabricate fuel pellets of UO.sub.2 alone or UO.sub.2 containing an oxide of plutonium, gadolinium or erbium consists of the following steps ; mixing or homogenizing fresh fuel powder, cold-pressing powder into green pellets, sintering green pellets in a reducing atmosphere at a temperature of at least 1500.degree. C., to achieve density of about 95% TD (theoretical density is 10.96 g/cm.sup.3), and then grinding sintered pellets to a diameter set by specification.
Nuclear fuel pellets used in nuclear power reactors must meet stringent fuel specifications in order to allow efficient and economical operation of the power reactors.
During sintering, some pellets have cracks or deform into an "hour-glass" shape having a central diameter too small for use, and sludge is also produced during grinding. Pellet chips can be produced during handling. In addition, the corresponding batch will be sometimes defective pellets if the sample representing one batch does not meet fuel specification. Since defective fuel pellets and grinding sludge are expensive and radioactive, they can not be discarded. Under a normal operation of fabrication, plant fuel scrap is recycled into the manufacture of new fuel pellets.
The art commonly used in recycling UO.sub.2 fuel scrap is known. Defective UO.sub.2 pellets are comminuted to U.sub.3 O.sub.8 particles through the oxidation at a temperature in the range of 400.degree. C. to 700.degree. C. for 2 to 4 hours under a flowing air. Recycled U.sub.3 O.sub.8 particles and grinding sludge are mixed with fresh UO.sub.2 powder, and then the mixed powder is pressed and sintered to produce new UO.sub.2 fuel pellets. However, recycled U.sub.3 O.sub.8 particles and grinding sludge are much less sinterable than fresh UO.sub.2 powder, and, in particular, especially recycled U.sub.3 O.sub.8 particles cause more complicated problem since the reduction of U.sub.3 O.sub.8 to UO.sub.2 makes pores in the fuel pellet during sintering in a reducing atmosphere. It is known that the sintered density of fuel pellet decreases with the contents of recycled U.sub.3 O.sub.8 particle and grinding sludge, so the amounts of recycled U.sub.3 O.sub.8 particle and grinding sludge which can be directly mixed with fresh UO.sub.2 powder are limited within about 7% and about 3% by weight, respectively, in order to avoid excessive density drop.
In case a large amount of defective UO.sub.2 pellets has to be recycled, the prior art described above will not be an effective method since the amount of recycled U.sub.3 O.sub.8 particle which can be directly mixed with fresh UO.sub.2 powder is restricted, which means that it will take a long time for defective pellets to be recycled. Moreover, the remaining defective pellets will have to be stored for a very long time, if new fuel pellets of which U.sup.235 enrichment is different from that of scrap material are fabricated before all the defective UO.sub.2 pellets are recycled. Recently, the amount of recycled U.sub.3 O.sub.8 particle to be added to fresh UO.sub.2 powder is much restricted since fuel density is controlled to be in a higher range within fuel specification.
The problem that the amount of fuel scrap which can be directly recycled is limited is common to the manufacture of not only UO.sub.2 fuel but also UO.sub.2 fuel containing an oxide of plutonium, gadolinium or erbium. When UO.sub.2 fuel containing an oxide of plutonium, gadolinium or erbium is produced, this problem is much aggravated since these fuels are less sinterable than UO.sub.2 fuel. Thus the amount of fuel scrap which can be directly recycled will be much smaller.
In order to overcome the above problem, the arts have been disclosed in which fuel scrap is treated entirely to be sinterable powder suitable for the manufacture of new fuel pellets. U.S. Pat. No. 3,578,419, U.S. Pat. No. 3,294,493, U.S. Pat. No. 3,140,151, U.S. Pat. No. 3,343,926, and European Pat. No. 84 129 describe methods of processing fuel scrap through oxidation and reduction in a fluidized bed; fuel scrap consisting of defective pellets and grinding sludge is oxidized to U.sub.3 O.sub.8 particle, which is then reduced to UO.sub.2 particle, and the UO.sub.2 particle so produced is subsequently oxidized and reduced up to 5 times. Particle size becomes smaller due to the repeated oxidation-reduction cycle, and thus sinterable UO.sub.2 powder can be produced in at least one oxidation-reduction cycle. Specific fluidized beds and processing variables such as temperatures for oxidation and reduction and/or gas composition are disclosed. However, disadvantages of the above art that a fluidized bed has to be additionally provided and powder treatments are hard to control. Oxidation rate of UO.sub.2 powder is much faster than that of defective UO.sub.2 pellets and thus resultant reaction heat can increase the temperature of powder above 800.degree. C. The powder so produced has unexpectedly a very low surface area and thus is poorly sinterable.